Randomization
Most, if not all games, contain some element of randomization. I’m talking games here, all games, not just video games. Ball bounces, dice rolls, card shuffles, are all complex enough equations that, as far as the average human brain is concerned, they are effectively random. Often, as is the case with chess or checkers, the random element is provided by a second human. Maybe this is not so much “random” as it is simply the obscured or inscrutable tactics of another human player. The end result is the same. We simply don’t know what will happen moment to moment, and we need to react to events as they unfold. Successfully predicting, or reacting to a random event makes people feel excitement, elation, a sense of accomplishment. Otherwise known as fun.